I tried the taller tire thing. My neon had factory 185 65 R14 tires. When it came time to replace I put on 185 70 R14's. I noticed an instant decrease in power. Which will end up being worse FE. But once I maxed out the tire pressure my FE was right back where it was before. So I guess I'm saying it doesn't help any. Not for my ride anyway.
2000 Plymouth Neon
Did you correct for introduced odometer error when calculating your MPG with the bigger tires? Bigger tires would cause the odo to under-report distance travelled, which would make it appear you're getting worse fuel economy in your calculations.
If your calcs (uncorreted) show the same MPG as before the swap, you're actually doing better.
Cool , I did use a tire calculator on the net to see the speedo difference but I had forgotten about the difference. It says I am going 67.5 mph when the speedo says 65 mph. So I guess bigger is better.
2000 Plymouth Neon
I agree with badgett. The tire size 185-65-R14 means, the width is 185 mm, 65 means the height of the tire is 65% of 185 mm, R14 means wheel size is 14 inches. Therefore if you have such a tire, you can only change it to 185-70-R14 or 185-75-R14 depends on the clearance between the tire and body unless you want to change wheels. The rpm is engine rpm, odometer and speed meter are programmed with the rpm meter, therefore you cannot notice before you take out your calculator. If you changed your tire from 65 to 70, the actual tire diameter will be (185x0.70+14x25.4)/(185x0.65+14x25/4)=1.02 or just 2 % increase.
So if you see 65 mph and 3,000 rpm in your meter, you are actually 66.3 mph. If you travelled 100 miles, it is actually 102 miles. Some auto dealers may void your warranty if you have done so. If you want actual 65 mph, you should drive 2,942 rpm and/or 63.7 mph. Running lower rpm usually gives better mileage. Anyway the change will be small.
I'm waiting for my tires worn out, I'll post my own result when it comes.
I've thought about regearing the fifth for a higher ratio (lower RPM).
I've also over-diametered the tires to lower revs per mile and have come to the conclusion that this change (lower revolutions per mile) is not a benefit to me. Here's why:
I tend to do a lot of suburban and rural secondary road travel. My speed is infrequently above 50 mph. My trip meter display of average speed is closer to 35 mph. With the factory gearing and factory size tires I can travel at that speed at idle rpm in 5th. With the larger tires, or if I swapped for a taller 5th, I'd have to drive faster (traffic conditions don't permit that) or drive in 4th at some rpm faster than my present. Either of these will hurt, not help, my fuel economy.
If I were to drive at the original poster's 75 mph it'd be obvious that I wasn't truly interested in reducing fuel consumption. Driving 75 instead of 55 saves just over 36% of the travel time but increases air drag by 85%!
I put on taller tires and a differential with a taller ratio. It didn't have much effect on FE unfortunately. At least FE didn't get worse. It did have some nice effects I hadn't anticipated. I could go 70 with ease (which didn't help FE!) where before the engine was revved up a little high at those speeds. Most of all, the engine noise was considerably reduced. And finally, the engine ought to last a bit longer.
Where taller really helps is if the car has power to spare. Being British, my car was already underpowered, so that worked against the idea. (Britian had an engine tax that encouraged manufacturers to put in the smallest engine that could move the car. And the engines were aways 1 or 2 cc less than some round number.) If you improve your power with a few simple techniques like changing to lighter oil, LRR tires inflated to the max, and perhaps an aero mod, then you're in a better position to reap benefits from taller tires. How does your car handle hills? If it can maintain speed without straining while going uphill, then probably taller will help. My engine otoh is currently in bad shape, and as it was already underpowered when up to snuff, the taller ratio hurts more now that it's putting out something like (wild guess) 80% of the power it ought.
Also, taller magnifies your gas consumption both directions. That is, a driver who burns excessive gas thanks to hard driving with lots of acceleration, jack rabbit starts and such, will do even worse with taller ratios. Drive easy, letting the car lose a little speed on the ups and getting speed back on the downs, and taller will aid you.